1. Field of the Invention
This invention relates to a device for monitoring the time-temperature history of an article in which a vapor is allowed to permeate through a porous substrate contacted with a solution of a polydiacetylene indicating compound in a suitable solvent, which undergoes a color change upon contact with the vapor, thus forming a moving boundary as vapor permeates through the substrate.
2. Brief Description of the Prior Art
A host of perishable products such as frozen foods, blood, vaccines and the like require a means for monitoring time temperature shelf-life histories. Various indicators have been described in the prior art as meeting this need with varying success.
Indicators are known that use a liquid or liquid vapor for activating a color response when a predetermined shelf life has expired. For example, U.S. Pat. No. 3,844,718 discloses a defrost indicator which is activated by the contact of water or water vapor with a water-soluble ink supported on a hydroscopic substrate.
U.S. Pat. No. 3,768,976 discloses a time-temperature indicator that depends upon the rate of permeation of oxygen through a polymer envelope containing an aqueous solution of a red redox dye. Upon oxidation, the red dye turns colorless, indicating that the perishable has been exposed to too high a temperature for too long a time.
U.S. Pat. No. 3,915,719 discloses a temperature indicator in which a frozen liquid is separated from an indicating layer by a liquid-soluble barrier. When the frozen liquid thaws, a time delay is introduced by the rate of dissolution of the liquid soluble barrier. Only upon dissolution of this barrier does color indication occur.
U.S. Pat. No. 3,501,297, discloses a mixture of diacetylenes, irradiated by ultraviolet radiation producing a blue-bronze color, capable of being converted by contact with warm ethanol vapors to a red color. However, such a color change by itself, is insufficient to suggest application as a practical time-temperature history indicator, since the color change may simply indicate that a particular temperature has been exceeded, without an indication either of the length of time that temperature has been exceeded or of the time-averaged exposure at high temperatures.
U.S. application Ser. No. 911,565 (Patel to Allied Chemical, 1978) describes a device for measuring time-temperature histories of an article in which a vapor is allowed to permeate through a porous substrate coated with an indicating solid thus producing a color response upon contacting said solid. Due to the fact that the vapor front permeates through and along the substrate, a moving boundary between two colors is created and advances as a function of time and temperature. However, total lifetimes are generally short for such devices, e.g. about a few hours to a few days and devices with longer lives are needed for monitoring articles having relatively long shelf lives.
U.S. Pat. No. 4,042,336 (1977) describes a device for monitoring time-temperature histories comprising a gas generating means, an indicator means supported on a wick, and a rate controlling means for the transmission of the gas generated to the indicator means in which it is possible to alter the time span over which the device is functional by incorporating into the wick a quantifier reactive with said gas. However, the device requires a quantifier as an integral part of the indicator system and does not mention or suggest the use of polydiacetylenes as the colorindicating material.